This invention relates to a method and apparatus for cutting sheet material and is concerned more particularly with problems that arise in automated cutting machines when a plurality of pattern pieces in a closely packed array are cut from limp sheet material.
Numerically controlled cutting machines such as disclosed in U.S. Pat. No. 3,495,492, having the same assignee as the present invention, are well known in the art and are widely accepted in industry for cutting various limp sheet materials such as woven or nonwoven fabrics, vinyl and other plastics, paper, cardboard, leather and others. The machines frequently are used to cut pattern pieces of various shapes in a predefined marker. A marker is an array of pattern pieces arranged as cut in closely spaced and sometimes contacting relationship in order to minimize the total quantity of material used. In the automatically controlled cutting machine of U.S. Pat. No. 3,495,492, a reciprocating cutting blade is guided along cutting paths defined by the pattern piece peripheries by means of a numerical or other control that responds to program data defining the shapes and positioning of the pattern pieces in the marker array.
A phenomenon that exists in cutting limp sheet materials in contrast to cutting in other arts is the fact that a blade having a sharp leading knife edge severs the material as the blade advances along a cutting path but does not remove material to any significant extent. As a result the material is pushed aside by the advancing blade and generally flows around the cutting blade in pressing engagement. Because of the engagement of the blade and material, and also because limp sheet materials are pliable even in a multi-ply layup compacted by vacuum as disclosed in the referenced patent, significant forces can be developed against the blade and cause the blade to depart from the programmed line of cut regardless of the accuracy with which the blade positioning mechanism is operated.
In cutting multi-ply layups of sheet material with a cantilevered knife blade, pattern pieces cut from the upper plies of the layup may have slightly different shapes and dimensions than the same pieces in the lower plies where the disturbing forces applied to the blade by the material cause the blade to bend. Such forces and the resulting bending are attributable to a number of factors, some of which are known and others of which are unknown. However, it is known that the forces frequently arise in connection with points of tangency or close approach in a closely packed marker array. When a cutting blade passes in close proximity to an adjacent pattern piece that was cut at an earlier stage in the operation, the kerf created by the previous cut interrupts the continuity of the limp sheet material and allows the material at one side of the knife blade to yield more easily to the blade than at the opposite side. As a result the blade experiences unbalanced lateral loading. Naturally, the closer the cutting path approaches a previous cut, the greater the unbalanced loading will be on the blade and the greater the blade bending. The blade may eventually break or jump completely into the kerf of the previous cut. Inaccuracies or damage to the machine are the ultimate consequences.
Several techniques have been developed to overcome the difficulties that are associated with tangencies and points of close approach in marker arrays. U.S. Pat. Nos. 3,855,887 and 3,864,997 having the same assignee as the present invention, reveal that a reciprocated knife blade may be slowed down with reduced feet rate signals in such critical cutting areas, and yaw signals may be applied to rotate the blade out of a tangent position at the same time. Until now, however, the introduction of compensating or remedial commands such as the reduced feed rate and yaw signals was left to the experience and skill of the person who digitized the marker array and prepared the cutting program in a basically manual process.
It is, accordingly, a general object of the present invention to provide method and apparatus for automatically preprocessing data defining a marker to identify tangencies, points of close approach and other critical cutting conditions, and to develop compensating or remedial commands for guiding a cutting blade past such cutting conditions without sacrificing accuracy or damaging the machine due to excessive blade loading.